Queue in SQL

Hi!

Question:
I need to store some incoming data and retrieve them one by one (LIFO).
Different processes will manage the storage and the retrieval.
How can I retrieve only the first row from a table?
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Explanation
I intend to use libpq and C. as far as I know to retrieve the result from a
query I need to do the following:

BEGIN A TRANSACTION
DECLARE CURSOR cursor FOR SELECT
FETCH one row from cursor
Process
CLOSE cursor
DELETE row
COMMIT

Let say I have 100,000 records in my table, I have to pick up only the first
row. I do not have any special criteria to execute a query, and I only need
the oldest added row (let say the first row if the table is indexed).
How can I narrow the search criteria, not to receive all of the 100,000
record in the result?
Unfortunately, I do not know the internal working of the SELECT. I think it
should create a temporary object to store the result. This object size may
depend on the number of the rows in the result and the size of the stored
data per row. I can fetch the rows one by one using this temporary object.

Kind Regards,
Gyorgy Molnar